1. Field of the Invention
The present invention relates to an apparatus for driving a liquid crystal display (LCD) apparatus, and more particularly, to a low power source driver for use in the LCD driving apparatus.
2. Description of the Related Art
Since LCD panels are thinner in size and lower in power dissipation as compared with cathode-ray tube (CRT) panels, the LCD panels have recently been applied to personal computers, word processors, color telereceivers. Particularly, since active matrix-type LCD apparatuses have a high-speed response, a fine screen with a high quality, and a multi-gradation display, the active matrix-type LCD apparatuses have been in demand.
Generally, an active matrix-type LCD apparatus is constructed by a semiconductor substrate having thin film metal wire, a transparent pixel electrodes and thin-film transistors (TFTs), a counter substrate having a transparent common electrode, and liquid crystal inserted between the semiconductor substrate and the counter substrate. A gradation voltage is applied to each pixel electrode by controlling the TFT with a switching function, and transmittance of the liquid crystal is changed by the difference in voltage between each pixel electrode and the common electrode to provide display on the screen.
Provided on the semiconductor substrate are data lines for applying gradation voltages to the pixel electrodes and scan lines for applying switching control signals (scan signals) to the TFTs. Then, when the, scan signal of the scan line is at a high level, all the TFTs connecting the scan line are turned ON, and the gradation voltages sent to the data line are applied to the pixel electrodes through the TFTs. When the scan signal becomes low to turn OFF the TFTs, the difference in voltage between each pixel electrode and the common electrode is maintained until the next gradation voltages are applied to the pixel electrodes. Thus, when scan signals are sequentially sent to each scan line, gradation voltages are applied to all the pixel electrodes, so that display on the screen is renewed at every frame period.
An LCD driving apparatus for driving the data lines is required to charge/discharge a large load of each data line including a liquid crystal capacitance, wiring resistances and wiring capacitance.
An LCD driving apparatus is generally constructed by a voltage divider, a decoder and driver connected to a data line. Conventionally, the driver is implemented by operational amplifier (see: S. Saito et al., “A 6-bit Digital Data Printer for Color TFT-LCDs”, SID 95 Digest, pp. 257–260, 1995). Since the operational amplifier has a high current supplying capability, the driver can drive the data line having a large capacitance load at a high, speed. Additionally, even when the threshold voltages of transistors within the operational amplifier fluctuate slightly, the fluctuation of the output voltage of the operational amplifier is relatively small. In addition, the output voltage can be highly accurate.
In the prior art driver, however, the number of operational amplifiers with a large number of elements increases with the number of data lines. Therefore, if an LCD driving apparatus using the prior art driver is constructed in the form of a single integrated circuit device, the size of the integrated circuit device must be increased to accommodate enough operational amplifiers thereby increasing the manufacturing cost thereof. In addition, steady currents are required for the operational amplifiers, which increases the power dissipation. The structure is not suitable for use of low power loss. The detailed technology for employing the operational amplifier in an LCD driving apparatus can be found in U.S. Pat. No. 6,075,524, issued to Ruta, entitled “Integrated Analog Source Driver For Active Matrix Liquid Crystal Display”. U.S. Pat. No. 6,127,997, issued to Tsuchi, entitled “Deriver For Liquid Crystal Display Apparatus With No Operational Amplifier” discloses another LCD driving apparatus which is constructed without the operational amplifier. However, there is still a problem of larger channel precharge charge loss since a large swing of charging or discharging operation is carried out in the structure.